The present invention relates to a projecting screen for optical purposes, which fulfills the stringent requirements for the reproduction of macroscopic as well as microscopic pictures in transmission operation. The invention has many possibilities of application in the construction of scientific instruments, especially where highly accurate evaluation of the information reproduced on the screen is required.
As a rule, projecting screens comprise surface optical components which scatter the traversing light due to mechanical processing of their surface or appropriate formation of their surface. Projecting screens have also been known that have been made light-scattering by means of a matrix of scattering particles provided throughout its entire volume. A high yield of scattered light, hereinafter called transmission, and a high resolution for reproducing small details are additionally of great importance for the performance of projecting screens besides a scattering capacity. Although these requirements can largely be met by the known projecting screens, a substantial flaw exists in that the projected reproductions are not free from grain structures and flickering or scintilation effects. The latter effect becomes physiologically disturbing, especially on extended subjective viewing of the projected images and also affects recognition of the fine reproduction details. This results from the fact that the light traversing the projecting screens, besides scattering, is also subjected to refraction and reflection on the surfaces of the scattering structural elements.
In order to prevent flickering or scintillating effects as well as grain structures, a projecting screen arrangement is known comprising two scattering media which are in direct surface contact or are arranged opposite each other with a small air gap in between, which are slowly moved relative to each other in rotation or in translation (GB-PS No. 592,815). The disadvantage of this arrangement is that transmission is worsened. Since vibration of the whole arrangement caused by the motion processes cannot be prevented, the resolution is also lowered.
To overcome these flaws, the prior art has come up with projecting arrangements in which two media having light-scattering surfaces execute an oscillating linear movement in relation to each other through electromagnetic excitation (AT-PS No. 195,666; US-PS No. 4,143,943). Not considering the relatively high costs of such equipment, the main flaw of these arrangements is that the flickering or scintillating effects cannot be completely prevented. Depending on the corresponding amplitude of the oscillation, resonance conditions are formed which lead to the formation of zones in the projected images where the flickering or scintillating effects as well as grain structures appear as before and reduced resolution persists. The resonance conditions may also trigger vibration effects in the projecting screen arrangements which hamper evaluation of the images. The prior art has also proposed inserting two scattering disks with different scattering capacities, wherein only the less scatterable disk is movable with movement carried out at relatively low speeds or amplitudes (DE-OS No. 1,945,486; DE-OS No. 2,138,340). Although the flickering and scintillating effects can largely be excluded in this manner and the resonance and vibration conditions of the whole arrangement can be prevented, this type of projecting screen has insufficient transmission qualities and a yet too reduced resolution. A projecting screen arrangement has also been known in which a mixture of scattered particles and air is blown through past behind a scatter disk (US-PS No. 2,906,169). Not considering the high expenditures with equipment, this arrangement does not result in uniformity of the optical effects over a larger viewing field for scientific evaluation of the information represented on the screen and is also not feasible. Such an arrangement is not appropriate for high resolutions. Transmission qualities are insufficient.
Finally, the prior art discloses a transmitted light projection system which corresponds to a large liquid crystal cell in regard to its arrangement (DE-OS No. 2,138,118). By providing an electric field, it is possible to practically remove the flickering or scintillating effect and to create a grain-free image. Substantial disadvantages arise, however, from the limited life span of such systems. Moreover, their operational efficiency depends largely on the temperature so that substantial additional expenses have to be incurred with equipment. The indicated resolution of 20 linear pairs per mm is not sufficient in microscopic requirements.
All known technical solutions have the common flaw that the flickering or scintillating effects and the grain structure cannot be effectively removed without negatively affecting the other extremely important qualities, such as resolution and transmission.
The object of the present invention is to create a projecting screen for optical purposes in which the flaws and disadvantages of the prior art can be avoided. In particular, a projecting screen is created which ensures high quality and adherence to detail in the reproduction and evaluation of the projected images, moreover, making viewing for extended periods possible without problems.